Considerable evidence supports the concept that the pulmonary damage produced by exposure to high levels of oxygen is mediated to a large extent by oxygen free radicals, the formation of which is catalyzed by iron. For this reason, histologic and ultrastructural studies were made to determine whether or not ICRF-187, a chelator of iron, has a protective effect against the development of pulmonary damage in rats exposed to pure oxygen. The results obtained showed that ICRF-187 has a marked protective effect against these lesions. Such an effect may provide a basis for the clinical use of this agent in the management of hyperoxia.